An implantable stimulation lead is a medical device that delivers stimulation pulses from an implanted pulse generator to the heart, or other body tissue, for the purpose of causing a desired muscle contraction. For cardiac muscle stimulation, such lead is typically inserted through one of the main veins of the patient, e.g., the superior vena cava, so that a distal end of the lead may be directed inside the heart. Electrodes positioned at the distal end of the lead make contact with the cardiac tissue. Implantable stimulation leads may be classified as being unipolar (having a single tip electrode), bipolar (having a tip electrode and a ring electrode), or multipolar (having three or more electrodes).
As used herein, the distal end of the implantable stimulation lead is that end which makes electrical contact with the heart and the proximal end is that end which is connected to the pacemaker through a connector top. Hereinafter, the proximal end of the implantable stimulation lead will be referred to as the "proximal connector assembly". The proximal connector assembly typically takes the form of a male connector, with the pacemaker connector top taking the form of a female connector. When joined, good electrical contact must be maintained between the terminals of the proximal connector assembly and an appropriate feedthrough terminal of the pulse generator housing. Furthermore, such lead connection must be secure, so that it does not disconnect during use, yet detachable in the event the pulse generator or lead needs to be replaced. Moreover, such connections must at all times remain insulated and sealed from ionic body fluids, which body fluids are conductive and could cause an electrical short.
With the arrival of dual chambered pulse generators, it was necessary to have two female connectors within the connector top to accommodate two leads. Thus, it is preferable for bipolar leads to have an "in-line" lead assembly (as opposed to a "bifurcated" lead assembly) to minimize the height of the connector. For example, U.S. Pat. No. 4,951,687 (Ufford et al.) and U.S. Pat. No. 4,572,605 (Hess) show in-line bipolar proximal lead assemblies with and without sealing rings, respectively, made using conventional techniques. That is, these bipolar leads included a pin terminal and a ring terminal coupled to a distal tip and ring electrode, respectively, by coaxial conductors. The electrical connection to the pin terminal is made by isolating and stretching the inner coil of the coaxial conductor over the pin terminal and either crimping or welding it thereto. The electrical connection to the ring terminal is similarly made by isolating and fixturing the outer coil of the coaxial conductor onto the ring terminal and crimping or welding it thereto. Once connected the whole assembly is then injection molded in a body compatible material. However, many of the existing methods and techniques are no longer suitable for the smaller pulse generators and leads that are currently being used.
Currently, there has been a tremendous demand to incorporate physiologic sensors onto the implantable stimulation lead. These sensors will measure a variety of physiologic parameters, such as, blood oxygen saturation, blood pressure, ejection time, pH, temperature, impedance, heart wall motion, etc. The additional sensor typically requires two additional conductors and proximal terminal contacts. However, the need for additional terminal contacts also requires additional space for isolation and for enabling the crimp or weld procedure.
In anticipation of these sophisticated leads, there have been several attempts at manufacturing "tripolar" leads, that is, leads having three distal electrodes. For example, U.S. Pat. No. 4,469,104 (Peers-Trevarton) shows a lead assembly having three spaced apart metal bands with resilient conductive rings on each band adapted to make contact with a corresponding ring terminal in the connector top. U.S. Pat. No. 4,469,104 (Doan et al.) also shows, in one embodiment, a tripolar lead assembly. However, neither embodiment is easily expandable to include four or more terminals. Furthermore, each of these lead assemblies are specially designed for each manufacturer's pulse generator connector top.
Furthermore, over the approximately 30 year history of the implantable pulse generator, a wide variety of techniques and methods have been used to connect leads to pulse generators. During that time, almost no standardization existed for their dimensions. While some manufacturers strongly believed that the sealing mechanism belonged on the lead in the form of seal rings, another group preferred to have the seal rings inside the pulse generator's connector top with a smooth sealing surface located on the lead. Great variability in the actual dimensions existed even with the standard "5 mm" or "6 mm" leads available from different manufacturers. As the pulse generator electronics and batteries become smaller, the connector system represents a larger percentage of the total pulse generator volume. Thus, many manufacturers are contemplating smaller connector systems.
To avoid a proliferation of new and incompatible designs, a major effort has been underway to standardize the interface between an implantable stimulation lead and a pulse generator. The proposed voluntary standard, known as VS-1, has subsequently been adopted by almost all pulse generator manufacturers worldwide. The VS-1 standard does not specify how a particular pulse generator connector must make contact with a implantable stimulation lead, it simply defines the dimensions of the 3.2 mm implantable stimulation lead and the dimensions of the corresponding pulse generator connector cavity into which the implantable stimulation lead is inserted. The VS-1 standard further specifies certain requirements as to leakage, conductivity and connect/disconnect force. For a further explanation of the VS-1 Standard, reference is made to "A Voluntary Standard for 3.2 mm Unipolar and Bipolar Implantable Stimulation Leads and Connectors," Calfee et al., PACE. Vol. 9, 1181-85 (Nov.-Dec. 1986), which reference is hereby incorporated herein by reference.
While the VS-1 standard advantageously represents a long needed movement towards industry standardization, the VS-1 standard disadvantageously restricts the dimensions of the proximal connector assembly of the implantable lead, which can in turn limit the number of conductors and terminals.
What is needed, therefore, is a multipolar implantable stimulation lead which meets the VS-1 standard, is easy to manufacture, and can be readily expanded to include many terminals and conductors without increasing its diameter and without excessively increasing the proximal connector assembly length (which in turn would affect the pacemaker connector top dimensions). It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.